1. Technical Field
The present invention related generally to imaging devices and, more particularly, to optical imaging devices with speed variable illumination.
2. Background
Optical imaging devices for electronically forming an image of an original are known. Optical imaging devices generally capture the image of an original and create an electronic representation of that image. Optical imaging devices typically comprise scanners, whether implemented in a scanner product, or whether incorporated into other appliances and products such as copiers, facsimile machines, multi-function devices, and the like.
Typically, the captured image provided by a scanner is a pixel data array that is stored in memory in a digital format. A distortion-free image requires a faithful mapping of the original image to the pixel data array. Scanners typically include at least one means for imposing a mechanical constraint during the image capture process in order to maximize the likelihood of faithful mapping.
The five types of scanners generally known in the art are drum scanners, flatbed scanners, sheet fed scanners, two-dimensional array scanners, and hand scanners. Drum scanners attach the original to the surface of a cylindrical drum that rotates at a substantially fixed velocity. During the rotation of the drum, an image sensor is moved in a direction parallel to the rotational axis of the drum. The combination of the linear displacement of the image sensor and the rotation of the original on the drum allows the entire original to be scanned. At any moment during the imaging process, the current position within the pixel data array relative to the original can be determined by measuring the angular position of the drum and the translational position of the sensor. The position of the pixel data array with respect to the original is fixed as long as the original is properly attached to the drum, the drum rotation is properly controlled, and the sensor is properly controlled in its displacement along the linear path.
Flatbed scanners include a linear array sensor that is moved relative to the original along an axis that is perpendicular to the axis of the array. Thus, the position of the sensor in one dimension may be known by tracking the relative movement of the sensor. The position of the sensor in the perpendicular direction is implicitly fixed by addressing a particular array element at which intensity is to be measured. In operation of a typical flatbed scanner, the original is placed on a transparent platen and the sensor, along with an image illumination source, is placed on a side of the platen opposite to the original. As long as the original is not moved relative to the platen, the pixel data array will be fixed with respect to the image to be captured.
Sheet fed scanners perform scanning by moving the original rather than moving the sensor. Precision paper transports provide a high degree of positional accuracy for the original during the image-capture process. The paper transports move the original over the stationary sensors to optoelectronically capture the image of the original. Such sheet fed scanning processes may be found in many types of facsimile machines.
Advantages of the drum, flatbed, and sheet fed scanners include the ability to accommodate documents at least as large as A4, or 8.5xe2x80x3xc3x9711xe2x80x3 paper. Moreover, some of these scanners can handle A1 paper in a single setup. However, these scanners are not generally portable, since they require a host computer for control, data storage, and image manipulation.
Two-dimensional array scanners may be used in the absence of mechanical encoding constraints, and require only that the array and the original be held motionless during an exposure period. A two-dimensional array of photosensitive elements directly accomplishes the mapping of the image of the original into a pixel data array. However, because a single 300 dpi mapping of an 8.5xe2x80x3xc3x9711xe2x80x3 original requires an image sensor having an array of 2500xc3x973300  elements, i.e. 8.25 million pixels, these scanners are cost-prohibitive in most applications.
Conventional hand scanners require a user to move a linear array of electrooptical sensor elements over an original. The movement is performed and controlled by hand manipulation. Array-position information is determined using methods such as those employed in operation of a computer xe2x80x9cmouse.xe2x80x9d As a linear sensor array is moved, the rotation of wheels, balls, or rollers that are in contact with the original is sensed, and the position information is determined from the mechanical details of the rotation. Some hand scanners now use additional navigational sensors instead of the mechanical means in order to sense and determine the details of the positional information (e.g., motion and rotation). When used in conjunction with stitching algorithms, the positional information allows hand scanners to handle larger-sized documents in multiple-passes. Stitching algorithms allow the hand scanner to join together multiple swaths of a larger document. Hand scanners are typically connected directly to a personal computer for image data storage, processing, and use. The stitching performed by these types of hand scanners is usually processed in the personal computer.
Another trait to consider with hand scanners, is the data rates available for the scanner technology. Data rates from the image sensor tend to limit the scanning speed. The scanners provide feedback to the user, typically by means of green or red light emitting diodes, to maintain the appropriate speed for the desired image resolution. Some hand scanners use electromagnetic brakes to prevent the user from dragging the scanner over the image too rapidly, or provide a mechanical element to add resistance against the original that increases with increases in scanning speed.
In many embodiments of hand scanners, the surface of the mechanical element in contact with the original has a high coefficient of friction, e.g. rubber, so as to resist slip and skid. A cylindrical roller or certain number of wheels connected by a rigid axle may be used to encourage a single translational degree of freedom during the scanning process. A straight-edge or other fixture is sometimes used to restrict the scan direction with respect to the original and to further encourage the translational constraint provided by the wheels or roller. Nevertheless, the position encoder approach (i.e., the sensing and determination of the details of the positional information, such as motion and rotation) is one that is often susceptible to slips and skips, so that the pixel data array loses its correspondence with the image on the original.
Although many hand scanners connect in some fashion to a personal computer for performing some of the processing, one hand scanner disclosed in U.S. Pat. No. 5,578,813, the disclosure of which is incorporated herein by reference, performs all processing and stitching completely within the scanner unit. In this disclosed embodiment of a hand scanner, two navigational sensors are generally used to detect motion in both x- and y-axes directions. Additional algorithms are also defined to compensate for any rotation that may naturally occur when the user is moving the scanner across an original. There is a tendency to impose some rotation as the hand scanner is moved across an original because of the natural positioning of the elbow as a pivot point. The rotation is likely to have a radius defined by the distance between the scanner and this elbow pivot point. As a consequence, the scanned electronic image would typically be distorted if not for the additional compensating algorithms.
The present invention is directed to a system and method for an optical imaging device comprising an image sensor for reading image data from an original slid over the image sensor, a variable power interface for varying an intensity of an illumination source corresponding to the image sensor, wherein the intensity is varied responsive to velocities of the original slid over the image sensor, and a processor for processing the image data.